Power/Performance Bits: May 5

CMOS-compatible laser
Researchers at Forschungszentrum Jülich, Center for Nanoscience and Nanotechnology (C2N), STMicroelectronics, and CEA-Leti Grenoble developed a CMOS-compatible laser for optical data transfer. Comprised of germanium and tin, the efficiency is comparable with conventional GaAs semiconductor lasers on Si.

Optical communications provide much higher data rates, and are beginning to look attractive for even short distances. “The most crucial missing component is a cheap laser, which is necessary to achieve high data rates. An electrically pumped laser compatible with the silicon-based CMOS technology would be ideal,” explained Detlev Grützmacher, director at Forschungszentrum Jülich’s Peter Grünberg Institute (PGI-9). “Such a laser could then simply be shaped during the chip manufacturing process since the entire chip production is ultimately based on this technology.”

Typically, III-V compound semiconductors are used to make lasers. “Their crystal lattice, however, has a completely different structure than that of silicon, which is a group IV element. Laser components are currently manufactured externally and must be integrated subsequently, which makes the technology expensive,” said Grützmacher.

By using germanium and tin, both group IV elements like silicon, the new laser can be manufactured during the CMOS process. “Pure germanium is, by its nature, an indirect semiconductor like silicon. The high concentration of tin is what turns it into a direct semiconductor for a laser source,” noted Dan Buca, working group leader at Jülich’s Peter Grünberg Institute (PGI-9).

The high amount of tin required is a problem. “A high tin content, however, decreases the laser efficiency. The laser then requires a relatively high pumping power. At 12-14 % tin, we already need 100-300 kW/cm2,” explains Nils von den Driesch of Jülich. “We thus tried to reduce the concentration of tin and compensate this by additionally stressing the material, which considerably improves the optical properties.”

The researchers reduced in content to about 5%, simultaneously decreasing the necessary pumping power to 0.8 kW/cm2. This produces very little waste heat, allowing the laser to be operated not only in a pulsed regime but also in a continuous working regime as a “continuous-wave laser.”

“These values demonstrate that a germanium-tin laser is technologically feasible and that its efficiency matches that of conventional III-V semiconductor lasers grown on Si. This also brings much closer to an electrical pumped laser for industrial-application that works at room temperature,” said Grützmacher. The new laser is currently limited to optical excitation and low temperatures of about -140°C.

Beyond data transfer, the team sees potential for the laser in infrared and night vision systems, gas sensors, and breath analysis.

Generating THz waves
Researchers at EPFL built a nanodevice capable of generating high-power terahertz waves. It can be mounted on a chip as well as a flexible substrate.

The device consists of two metal plates situated 20nm apart. When a voltage is applied, electrons surge towards one of the plates, where they form a nanoplasma. Once the voltage reaches a certain threshold, the electrons are emitted almost instantly to the second plate. This rapid movement enabled by such fast switches creates a high-intensity pulse that produces high-frequency waves.

The voltage can spike from 10V or lower to 100V in a picosecond. It can do this almost continually, emitting up to 50 million signals every second. When hooked up to antennas, the system can produce and radiate high-power THz waves.

The team’s device can generate both high-energy and high-frequency pulses. “Normally, it’s impossible to achieve high values for both variables,” said Elison Matioli, a professor at EPFL’s Power and Wide-band-gap Electronics Research Laboratory (POWERlab). “High-frequency semiconductor devices are nanoscale in size. They can only cope with a few volts before breaking out. High-power devices, meanwhile, are too big and slow to generate terahertz waves. Our solution was to revisit the old field of plasma with state-of-the-art nanoscale fabrication techniques to propose a new device to get around those constraints.”

“These nanodevices, on one side, bring an extremely high level of simplicity and low-cost, and on the other side, show an excellent performance. In addition, they can be integrated with other electronic devices such as transistor. Considering these unique properties, nanoplasma can shape a different future for the area of ultra-fast electronics”, said Mohammad Samizadeh Nikoo, a PhD student at the POWERlab.

The researchers say the THz generator could be installed in smartphones and other hand-held devices for wireless communication, and think the technology could have applications beyond generating THz waves.

Magnetic energy harvesting
Scientists from Pennsylvania State University and Virginia Tech propose a way to harvest energy from small magnetic fields with a device capable of generating enough electricity to power sensor networks for smart buildings or factories.

“Just like sunlight is a free source of energy we try to harvest, so are magnetic fields,” said Shashank Priya, professor of materials science and engineering and associate vice president for research at Penn State. “We have this ubiquitous energy present in our homes, office spaces, work spaces and cars. It’s everywhere, and we have an opportunity to harvest this background noise and convert it to useable electricity.”

The device is about 1.5 inches long and paper-thin, designed to be placed on or near appliances, lights, or power cords where the magnetic fields are strongest. It has a composite structure with magnetostrictive layers, which convert a magnetic field into stress, and piezoelectric layers, which convert stress, or vibrations, into an electric field. The combination allows the device to turn a magnetic field into an electric current.

The device has a beam-like structure with one end clamped and the other free to vibrate in response to an applied magnetic field. A magnet mounted at the free end of the beam amplifies the movement and contributes toward a higher production of electricity, the scientists said.

When placed 4 inches from a space heater, the device produced enough electricity to power 180 LED arrays, and at 8 inches, enough to power a digital alarm clock without charging a capacitor.

“The beauty of this research is it uses known materials, but designs the architecture for basically maximizing the conversion of the magnetic field into electricity,” Priya said. “This allows for achieving high power density under low amplitude magnetic fields.”

“In buildings, it’s known that if you automate a lot of functions, you could actually improve the energy efficiency very significantly,” Priya added. “Buildings are one of the largest consumers of electricity in the United States. So even a few percent drop in energy consumption could represent or translate into megawatts of savings. Sensors are what will make it possible to automate these controls, and this technology is a realistic way to power those sensors.”